1. Field of the Invention
The present invention relates to a light-emitting device, a flexible light-emitting device and a method of manufacturing the same. Further, the present invention relates to an electronic device and a lighting device on each of which the light-emitting device or the flexible light-emitting device is mounted.
2. Description of the Related Art
In recent years, technological development has been remarkably made in the field of displays. In particular, the needs of the market have stimulated tremendous progress in the technology directed to higher definition and reduced thickness.
In the next phase of this field, the focus is on commercialization of flexible displays, and a variety of proposals have been made for making displays flexible (for example, see Patent Document 1). Light-emitting devices using flexible substrates can be very light in weight compared with those using glass substrates or the like.
However, the biggest problem with practical application of such flexible displays is their lifetime.
This is because, as a substrate that should support light-emitting elements and protect the elements from moisture, oxygen, or the like in the surroundings, a glass substrate which is not flexible cannot be used and instead, a plastic substrate which is flexible but has high water permeability and low heat resistance has to be used. Since the heat resistance of a plastic substrate is low, a high quality protective film obtained by a high-temperature process cannot be formed. Moisture penetration through the plastic substrate adversely affects the lifetime of the light-emitting elements and further the light-emitting device to a great extent. For example, Non-Patent Document 1 introduces an example of the fabrication of a flexible light-emitting device in which a light-emitting element is formed over a substrate containing polyethersulfone (PES) as a base and is sealed with an aluminum film; however, the light-emitting device has a lifetime of about 230 hours, which is far from its practical use. Non-Patent Documents 2 and 3 each introduce an example of a flexible light-emitting device in which a light-emitting element is formed over a stainless steel substrate. In these examples, while moisture penetration through the stainless steel substrate is suppressed, moisture penetration from the light-emitting element side cannot be effectively prevented; therefore, the flexible light-emitting device is fabricated over the stainless steel substrate and a sealing film, in which layers of many kinds of materials are repeatedly stacked, is employed on the light-emitting element side so as to attempt to improve the lifetime.
Although a metal thin film such as an aluminum film or a stainless steel substrate has both flexibility and low water permeability, it does not transmit visible light therethrough with a normal thickness. Hence, in the light-emitting device, a metal thin film or a stainless steel substrate is used for only one of a pair of substrates between which a light-emitting element is interposed.